The present invention is directed to a pressure gage for measuring, indicating and controlling positive, negative or differential fluid pressures, and in particular to a pressure gage including a magnet coupled to a diaphragm, wherein movement of the magnet in response to pressure changes sensed by the diaphragm provides mechanical movement of a pointer to indicate the sensed pressure or pressure differential, and movement of the magnet creates a change in a magnetic field sensed by a Hall effect sensor wherein the output signal from the Hall effect sensor is connected to one or more switching relays for controlling associated devices in response to sensed pressure changes.
Pressure gages such as that disclosed in U.S. Pat. No. 4,347,744 of Dwyer Instruments, Inc., the assignee of the invention described herein, have been used to measure fluid pressure by the use of a flexible diaphragm that is coupled to a magnet. The magnet moves in response to changes in the differential pressure sensed by the diaphragm, and the movement of the magnet caused rotation of a double-flanged helix which in turn rotated the dial pointer of the gage to indicate the sensed pressure. The combination of a magnet and a Hall effect sensor has been used to sense changes in pressure in a differential pressure switch, such as in U.S. Pat. No. 6,089,098 of Dwyer Instruments, Inc. However, such differential pressure switches do not provide a mechanical reading of the pressure being sensed. Prior pressure gages have also detected when a sensed pressure is at or above, or is at or below, a selected pressure set point by the use of a photo interrupter that senses the position of the mechanical pointer of the gage. Such an arrangement requires a relatively complex and costly mechanical arrangement.
The pressure gage of the present invention is designed specifically for measuring low fluid pressures and small differential pressures. Such pressure gages are therefore occasionally subject to overpressure situations which can damage the diaphragm and other components of the gage. In addition, when a magnet is used in connection with a Hall effect sensor, an overpressure situation could cause the magnet to move past the Hall effect sensor to create a situation where the Hall effect sensor incorrectly indicates a sensed pressure or indicates that no pressure or differential pressure is being sensed. Consequently the Hall effect sensor may signal a switch to activate an associated device when it should not. There has consequently been a need to provide a gage wherein such consequences of an overpressure situation can be minimized or eliminated.